Friday, March 30, 2012

Select : Lock or No Lock?

Here is a post I happen to find today: why select command is not blocked by update command?

First, I thought it might be a transaction isolation level issue, but when I dig into the problem, something interesting pop up.

Here is the script to repro this issue
first create testing enviroment:
USE master;
-- =======================================
-- Create test database
IF DB_ID(N'db_xlock_test') IS NOT NULL
      ALTER DATABASE db_xlock_test
            ROLLBACK AFTER 0;
      DROP DATABASE db_xlock_test;
CREATE DATABASE db_xlock_test;
ALTER DATABASE db_xlock_test
-- =======================================
-- create table
USE db_xlock_test;
      id int IDENTITY
            PRIMARY KEY,
      name sysname
INSERT dbo.tb
SELECT TOP(50000) + N'.' + + N'.' +
FROM sys.objects O1 WITH(NOLOCK),
      sys.objects O2 WITH(NOLOCK),
      sys.objects O3 WITH(NOLOCK);

 Then, create a session to update the table
-- =======================================
-- session 1:create a transaction and update the table
      --update table
      UPDATE dbo.tb SET name = name
      WHERE id <= 2;
            spid = @@SPID,
            tran_count = @@TRANCOUNT,
            database_name = DB_NAME(),
            object_id = OBJECT_ID(N'dbo.tb', N'Table');
      -- display lock info
      EXEC sp_lock @@SPID;

 Finally, create another session to select the data from table, theoretically, the select will be blocked and the session will be in waiting status.

-- =======================================
-- session 2: select data
SELECT * FROM dbo.tb
WHERE id <= 2;
run session 2, and What happen?  we can still select data in session 2 without any blocking. The current transaction isolation level is “Read Committed”, why we can still select data out?

From the profiler trace we can find the session 2 only acquire IS lock on page and object which is not blocked  by session 1 - update command.

Then I reviewed the update command, the special is the “SET”

UPDATE dbo.tb SET name = name WHERE id <= 2;
so I changed the command with  
UPDATE dbo.tb SET name = 'a' WHERE id <= 2;
this time, the select command was blocked!!!! here is the profiler trace when session 2 was being blocked.
then I rollback the session 1 transaction,  get the new trace
so generally speaking,
1. when "SET name = name ", the select command doesn't request S lock on Key, only need IS lock on page.
2. when  "SET name = 'a' ", the select command request S lock on key(or page), so it was blocked.

but why? for first condition("name=name"), actually, the data is not changed, but for second condition("name='a'), the data is changed.

I did more testings:
rollback session 1 (SET name = 'a' ), run session 1 with "SET name = name " again, but this time, the session 2 was still blocked, even if we use "set name=name".
so I think if the data page is not touched(changed), SQL Server will use a smart way to get data without S lock, but once the data page is changed(even if we rollback the change), in order to avoid dirty data read, SQL Server will use S lock when selecting data, that make sense.
To prove my thought, after rollback session 1(SET name='a'), I run "checkpoint" to write the dirty page to disk, then do the testing with condition (SET name=name), this time the  select query return quickly and no blocking issue.

later, I searched on Internet, and found Paul White had shoot this issue before in a wonderful article:

The Locking Optimization

SQL Server contains an optimization that allows it to avoid taking row-level shared (S) locks in some circumstances. Specifically, it can skip shared locks if there is no risk of reading uncommitted data without them.

but sometimes the optimization is not smart, when I changed select query to
SELECT * FROM dbo.tb WHERE id <= 600

then blocking occurs no matter what update command is................



    FIFO has one disadvantage. It does not always allow the maximum concurrency. The lock manager in SQL Server 2005 allows as much concurrency as possible without starvation. Here is an example:

    Transaction T1 has an IX lock on table Foo. Transaction T2 runs a query on Foo and specifies the TABLOCK hint. T2 becomes blocked behind T1 because its S lock is not compatible with T1’s IX. Transaction T3 runs a select query on Foo without any hints – its IS request is granted immediately (i.e. before T2’s S request) because IS conflicts with neither IX nor S. However if transaction T4 attempts to run an update statement on Foo, it will become blocked behind T2 because its IX request is not compatible with T2’s S requests and because T2 made its request first, it has priority.

    In SQL Server 2000, both T3 and T4 would have been blocked behind T2’s request.

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